The chapter analyzes various definitions of language ecology (concentrating more on ecological aspects), ecolinguistics (more on linguistic aspects) and linguistic/language diversity (power relations are decisive for those whose definitions of language are valid). This is followed by definitions and ways of assessing biodiversity. The relationship between linguistic and biological diversities (biocultural diversity) is presented. Results of today’s ways of assessing all of them show that these diversities are rapidly disappearing. Causes for linguicide and ecocide and some parallels and differences between them are analyzed. Future trends and possible ways of counteracting this first-ever human-caused mass extinction of species and languages are discussed.